A conductance catheter is a medical device used to measure rapid, simultaneous changes in volume and pressure within the heart’s chambers. It provides detailed information about the mechanical function of the heart, offering insights into conditions like heart failure. The data allows for a comprehensive evaluation of the heart’s pumping action.
How Conductance Catheters Measure Volume
A conductance catheter functions by generating a low-amperage, high-frequency electrical field within a heart chamber, typically the left ventricle. The catheter is equipped with multiple ring-shaped electrodes along its length. The outer electrodes emit a constant current to create an electrical field, while the inner electrodes measure voltage differences within that field. These measurements are then combined to calculate the total chamber volume.
Blood is a highly conductive medium, and as the heart contracts and relaxes, the volume of blood inside the ventricle changes continuously. This fluctuation in blood volume directly alters the electrical conductance measured by the catheter’s electrodes. A greater volume of blood results in higher conductance, while a lower volume leads to lower conductance.
The raw conductance signal is not solely from the blood; the surrounding heart muscle also conducts electricity and must be accounted for. This is known as parallel conductance. To obtain an accurate blood volume, a calibration step is performed, often involving the injection of a small amount of hypertonic saline to transiently change the blood’s conductivity. This separates the blood’s signal from the muscle’s, ensuring the final volume measurement accurately reflects the blood in the ventricle.
The Pressure-Volume Loop
The primary output from a conductance catheter study is the pressure-volume (PV) loop, a graph that is considered a gold standard for assessing cardiac mechanics. This loop plots ventricular pressure on the vertical axis against ventricular volume on the horizontal axis over a complete heartbeat. Analyzing the loop’s shape, size, and position helps clinicians diagnose various cardiac conditions, including different types of heart failure and valve diseases.
The PV loop illustrates the four distinct phases of the cardiac cycle:
- Ventricular filling: The ventricle relaxes and fills with blood, causing volume to increase at low pressure.
- Isovolumetric contraction: The ventricle contracts, sharply increasing pressure without changing volume as all heart valves are closed.
- Ejection: The aortic valve opens and blood is pumped out, causing volume to decrease as pressure rises and then falls.
- Isovolumetric relaxation: The ventricle relaxes, pressure drops steeply, and volume remains constant until filling begins again.
Key Cardiac Function Metrics
One of the most fundamental metrics is Stroke Volume (SV), which is the amount of blood pumped out of the ventricle during one contraction. On the PV loop graph, the SV is represented by the width of the loop.
Another measurement is the Ejection Fraction (EF), which is the percentage of the ventricular blood volume that is ejected with each beat. It is calculated from the stroke volume and the end-diastolic volume. The PV loop also allows for the assessment of contractility, or the innate strength of the heart muscle.
The End-Systolic Pressure-Volume Relationship (ESPVR) is a measure derived from the top-left corners of multiple PV loops taken under varying conditions. The slope of the ESPVR line indicates the contractility of the ventricle, showing the maximum pressure the ventricle can generate at any given volume. Similarly, the End-Diastolic Pressure-Volume Relationship (EDPVR) provides information about the stiffness or compliance of the ventricle during filling.
The Catheterization Procedure
The placement of a conductance catheter is an invasive procedure performed in a cardiac catheterization laboratory. The process begins with the administration of a local anesthetic and sedation to ensure patient comfort. A small incision is typically made in the groin to access the femoral artery, though other entry points like the arm or neck can also be used.
Through this incision, a thin, flexible tube called an introducer sheath is inserted into the blood vessel. A guidewire is then carefully threaded through the sheath and navigated through the arterial system to the heart. Imaging equipment, such as fluoroscopy (a type of X-ray), is used to guide the wire and catheter into the left ventricle.
Once the guidewire is in place, the conductance catheter is advanced over it into the heart chamber. The physician can then begin recording the simultaneous pressure and volume data. After the procedure, the catheter is removed, and the patient is monitored for several hours, requiring bed rest for a period to ensure the access site heals.